When a user in a wireless communications network initiates a communication session, the nature of the communication may necessitate locating the user. Commonly, a user must be located when the user places a “9-1-1” emergency telephone call. Other scenarios exist as well. For example, a user may contact a service provider requesting directions to nearby restaurants or to a gas station. To provide accurate information, the service provider must be able to determine the user's location.
The advent of cellular telephones required new locating technology. Previously, 911 calls originated mainly from landline telephones. Therefore, the process of determining the location of a caller was usually a simple matter of looking up the street address of the calling telephone number. Unlike landline telephones that were fixedly associated with particular street addresses, cellular telephones could place calls from anywhere within a cellular coverage area. Thus, a more complex system of position determination was required in order to determine the location of a calling cellular telephone.
To facilitate 911 services for cellular telephone callers, the United States Federal Communication Commission (FCC) mandated the implementation of “Enhanced 911” (“E911”) services. The E911 mandate was divided into two phases. According to Phase 1, the location had to be identified with an accuracy of at least cell and sector. As this information is typically maintained by a cellular wireless carrier in a subscriber's home location register (“HLR”), Phase 1 presented little technical challenge. According to Phase 2, the location must be provided with far more granularity than the cell and sector information maintained in the HLR. In response, the Telecommunications Industry Association (TIA) has proposed a standard entitled “Enhanced Wireless 9-1-1 Phase 2” or “J-STD-036-A” (including a recent addendum, J-STD-036-A-2002), the entirety of which is hereby incorporated by reference.
In order to achieve the accuracy specified by Phase 2, a service provider may employ a wireless position determining system that communicates with a wireless communication device (WCD), such as a cellular telephone, in order to determine its current location. The positioning system could be “handset-based,” in which case the cellular telephone would read its geographic coordinates from a satellite based positioning system (e.g., GPS) and report the coordinates over an air interface to the carrier. Alternatively or additionally, the positioning system could be “network-based,” in which case the carrier might employ a technique such as triangulation, (e.g., Advanced Forward Link Trilateration (AFLT) or Enhanced Forward Link Triangulation (EFLT)), to measure the telephone's location. Such network-based techniques may also be combined with handset-based techniques.
When a positioning system performs triangulation, the system determines the location of a WCD based on two or more fixed locations, such as the locations of base transceiver stations (BTSs). Triangulation can be carried out using various methods. In its most basic form, the angle of arrival of an incoming signal is measured at two or more stationary locations. Then, a system performing triangulation creates paths, extending radially at the angle of arrival, from each stationary location. The intersection of these paths is the estimated location of the device.
Recently, more sophisticated triangulation techniques have been developed, which use the delay between a WCD and two or more BTSs (preferably three BTSs) to locate a device. Specifically, the time a signal spends traveling over an air interface between the WCD and each BTS is measured and reported to a positioning system. Using that delay, along with the known speed of the signal, the distance between the WCD and each BTS can be calculated. Then, an arc with a radius extending the distance to the WCD is centered on each BTS. The intersection of these arcs provides the estimated location of the WCD.